1. Field Of The Invention
This invention is directed to motion controllers and compensators for items used in wellbores; to such devices useful with downhole drilling and/or milling tools and to downhole milling assemblies with such a device; to such devices useful with tools in a wellbore extending down from the sea floor and tools with such a device; and methods of using such items.
2. Description of Related Art
In milling a tubular with a downhole mill, using too much weight on the mill and/or advancing the mill too quickly can result in inadequate milling, inefficient milling, and damage to the milling system and to the item to be milled.
When milling in an environment in which contact between a milling system and a tubular to be milled is interrupted and then the milling system is again thrust against the tubular to be milled, it is possible to severely damage the milling system with an abrupt intense impact against the tubular being milled. In offshore milling operations, a mill lifted away from a tubular being milled, e.g. by a swell at the water's surface that lifts a boat or barge from which the milling system is suspended, can be slammed back into the tubular being milled as the swell passes and the boat is effectively lowered.
There has long been a need for apparatus to effectively control the rate at which an item is lowered in a wellbore. There has long been a need for an apparatus to compensate for unwanted motion that raises a tool in a wellbore when the tool is intended to be advanced in a direction opposite to that of the unwanted motion. There has long been a need, recognized now by the present inventors, to control the rate of advance of a downhole mill in certain milling operations. There has long been a need, recognized now by the present inventors, to advance, in a controlled manner, a downhole mill that is intermittently lifted away from an item that is being milled.